Question: Brad has a cylindrical barrel with radius 10 inches and height 15 inches.  He wants to fill it from a well, using a bucket in the shape of a hemisphere with a radius of 10 inches. How many trips must Brad make to the well in order to fill the barrel?
Answer: We start by finding the volume of the bucket and the volume of the barrel.  Let $r$ be ten inches.   The bucket is half of a sphere of radius $r$, so the volume it can hold is \[ \frac{1}{2} \cdot \frac{4}{3} \pi r^3 = \frac{2}{3} \pi r^3 . \]On the other hand, the barrel is a cylinder of radius $r$ and height $15 \text{ in} = \frac{3}{2} r$, so its volume is \[ \pi r^2 \cdot \text{height} = \frac{3}{2} \pi r^3 . \]Therefore the ratio of the volume of the barrel to the volume of the bucket is \[ \frac{(3/2) \pi r^3}{(2/3) \pi r^3} = \frac{9}{4}
= 2 + \frac{1}{4} . \]Thus two buckets will not suffice to fill the barrel, but three will, so Brad needs $\boxed{3}$ trips to the well.